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Parallel Rules

Parallel rules help draftsmen, surveyors, cartographers, architects, and navigators draw accurate parallel lines. The instrument comes in two main forms: two rectangular straight edges connected by brass or silver hinges, or a single frame surrounding a roller. The first type was known in Europe by 1600, while Englishman A. George Eckhardt is credited with inventing the second in 1771. The parallel rule was superseded for most uses by the T-square in the 19th century, but navigators continue to use parallel rules in conjunction with gridded charts.

The mathematics collections contain about twenty parallel rules and combination instruments, dating from the late 18th century to the late 20th century and ranging in length from 6 to 24 inches. The objects are made from ebony and other woods, brass, German silver, and plastic. They were manufactured in the United States, England, Italy, and Taiwan. They were used for military surveying, in navigation, in business, in art and technical drawing, and for placing handles on caskets. Several of the objects in this group illustrate innovations added to the basic instrument.

Acknowledgement

The digitization of this group of artifacts was made possible through the generous support of Edward and Diane Straker.

This 21" German silver hinged parallel rule has two small knobs for positioning the instrument. Brass pins secure the hinges. The top blade is marked: U. S. C. & G. S. NO. 323. A fleur-de-lis or letter H appears above the mark. The edges of the top blade are marked as a rectangular protractor, and the edges of the bottom blade are marked for nautical compass points.

Capt. William Andrew Field (about 1796–1871) of England added the protractor and compass scales to hinged parallel rules in 1854. This made it easier for ship navigators to move the rule without losing track of the ship's course. According to the accession file, the U.S. Coast & Geodetic Survey acquired this rule on July 23, 1919, and last issued it on March 18, 1920. Compare to MA*309661 and MA*309662.

This 12" clear plastic rolling parallel rule is combined with a scale of inches that is divided to 1/16"; a centimeter scale divided to millimeters; and a protractor divided to 5° and numbered by 30s from 0 to 90 to 0. Holes in the scales at every 1/2-inch and centimeter help the user align the instrument on a drawing. The holes may also be used with a pencil to produce parallel lines. The instrument is marked with a small centimeter-inch conversion table and the words: MADE IN TAIWAN.

The white roller is also marked with various useful mathematical formulas, a small table of sines and cosines of various angles between 1° and 45°, and comparative centimeter and inch scales that function as a line spacing guide for measuring evenly spaced lines.

The rule appears to be model 1205 of Hua Ching Manufacturing Co., Ltd., which has been making plastic and wooden drafting supplies in Taichung, Taiwan, since 1978. Several firms currently distribute the rule in the United States for a price of about $10.25.